I wasn't too scientific about this, but in a shaker, muddle 4 blackberries and 4 mint leaves. Add 1/2 cup of ice, juice of one lime, a shot or two (depending on how strong you like it) of tequila, spash of cointeau and about a teaspoon of agave nectar. Shake well and strain into a glass with ice. Top off with club soda and garnish with more fresh mint.